Investment Securities - Amortized Cost, Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,705,781	$ 1,558,787
Gross Unrealized Gains	47,126	13,596
Gross Unrealized Losses	(3,065)	(5,582)
Fair Value	1,749,842	1,566,801
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,946	50,952
Gross Unrealized Gains	1,732	12
Gross Unrealized Losses	0	(21)
Fair Value	52,678	50,943
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,994	28,982
Gross Unrealized Gains	545	280
Gross Unrealized Losses	(33)	(19)
Fair Value	27,506	29,243
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,982	78,116
Gross Unrealized Gains	667	540
Gross Unrealized Losses	(194)	(83)
Fair Value	74,455	78,573
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	86,532	90,662
Gross Unrealized Gains	1,052	902
Gross Unrealized Losses	(84)	(86)
Fair Value	87,500	91,478
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	160,514	168,778
Gross Unrealized Gains	1,719	1,442
Gross Unrealized Losses	(278)	(169)
Fair Value	161,955	170,051
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,406,084	1,248,814
Gross Unrealized Gains	42,390	11,234
Gross Unrealized Losses	(837)	(5,262)
Fair Value	1,447,637	1,254,786
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	61,243	61,261
Gross Unrealized Gains	740	628
Gross Unrealized Losses	(1,917)	(111)
Fair Value	$ 60,066	$ 61,778